This invention relates to a procedure for the production of steel band for the manufacture of parts fabricated by draw and ironing process, during which a hot rolled steel band is cold formed, in one or multiple stages, with a cold-rolling coefficient of at least 86%xc2x0, where at least one side of the band material is coated with a galvanic layer containing Ni, Co, Cu, Fe, In, Pd, Bi and/or their alloys, or with a roll-bonded cladding containing Cu and/or brass and/or their alloys.
Cold rolled steel band is used for the fabrication of rotationally symmetrical cold formed parts such as battery shells. The procedures applied during the cold forming are deep drawing and ironing, where the latter procedure is also called DI procedure (for drawing and ironing). Due to rising requirements as for the application and use properties of such steel band material, the industry seeks constantly improving mechanical properties and especially better forming properties. Good plasticity is characterized by high r values for anisotropy characterizing the deep-drawing quality, and by n values characterizing drawing and ironing properties, as well as by high stretching values. It is also advantageous if the forming properties are the same lengthwise, crosswise and diagonally, i.e., if they are isotropic. The advantage of isotropic properties of the steel sheet are substantially reflected in the uniformity of the material flow during cold drawing or drawing and ironing so that no or very little earing occurs which results in a reduction of metal sheet waste.
In order to achieve an almost isotropic forming, steel sheet with very small permissible thickness variations in a texture-free and homogeneous rolled band or sheet is used.
The undesirable earing and its causes are explained in detail in the magazine xe2x80x9cBlech, Rohre, Profilexe2x80x9d [Metal Sheet, Tubes, Profiles], September 1977 issue, on pages 341 through 346. The same article also describes that an earing-free material can normally be produced only by normalizing (annealing for relieving stresses) in a continuous annealing furnace at a temperature of about 1000xc2x0 C. However, the operation of a continuous annealing furnace at such a high temperature requires high investment and operation costs.
DE-38 03 064 C1 reveals that low values for anisotropy and therefore a low tendency to form earing is achieved for globular-type steels that the steel has a higher content of titanium of up to 0.04% using a cold rolling coefficient over 80%. However, such high rolling coefficients reach the stretching limit of steel of over 250 N/mm2. In addition, steels stabilized by an ingredient of titanium are known to require high recrystallization temperatures, which would lead to a high tendency of individual band layers to stick together if such a steel band should be annealed in coiled state. However, the resulting damage of the steel sheet surface is very undesirable for high-value products and thus would result in a high rate of rejected products.
The application of a continuously operated band annealing furnace for the production of steel sheet designed for the fabrication of parts manufactured by drawing and ironing is also revealed in the publications U.S. Pat. No. 5,078,809, WO 98/06881 and EP 0 822 266 A1. The latter document describes steel with a low content of carbon, whose steel analysis further contains boron with a content between 0.0005 and 0015 weight %. The aforementioned lower limit is based on the requirement to increase the resistance of the steel sheet to corrosion by adding boron to the steel melting charge. The document EP 0 822 266 A1 justifies the upper limit of 0.0015 weight % by the circumstance that a higher boron content would cause forming defects in cylindric parts.
The document DE 20 19 494 A describes a procedure for the production of corrosion-resistant coated steel. A coating of at least one metal from the group Co, Cu, Ni and Ti is applied on a pickled, hot rolled steel band, and the hot rolled steel band, with the coating on it, is then cold reduced to final size. During the one or several stage cold reduction process, a reduction coefficient of about 90% and more can be achieved. The cold reduced steel band is then annealed for recrystallization, where the annealing is preferably performed in a continuous annealing procedure. In case only one annealing step is required, it can be done by means of a box annealing procedure, where a temperature in the range between 566xc2x0 C. and 621xc2x0 C. should be maintained for a time period of 1 to 5 hours. The goal of such procedure is to prevent extensive formation of an alloy of the metal in the coating and the underlying band steel during the vapor-depositing of the coating. An exemplary composition of the steel plates entering the manufacturing process is: 0.035% C, 0.49%, 0.10% P, 0.11% S and 0.035% Si. This document does not mention a possible content of boron.
The document GB 2 101 156 A describes a procedure for the production of a steel band for deep drawing. The procedure described in this document includes conventional hot rolling and cold rolling steps applied to an aluminum-killed steel. The steel used according to this document contains no more than 0.007% nitrogen and such a quantity of boron that corresponds with a boron to nitrogen ratio of 0.5 to 2.5. In the provided examples the actual quantity of boron is between 0.0025% and 0.0040%. According to this document, any annealing of the steel band is performed exclusively in the form of a continuous annealing procedure.
The document JP-A-2 267 242 describes a procedure for the production of a cold rolled steel band made of aluminum-killed steel with a very low content of carbon. In order to chemically bond the nitrogen contained in the steel, aluminum is added to the starting steel material, which will then chemically bind the nitrogen during the subsequent hot rolling process to form aluminum nitride. After the following pickling and cold rolling procedures the steel band is annealed in a box annealing procedure. According to this document, the steel band does not have any coating, and the steel does not contain any boron.
Finally, the document DE-195 47 181 C1 describes a type of steel with content of titanium, vanadium, or niobium, where a sort of a mixed-grain steel material is achieved based on certain hot rolling conditions under the gamma range of the iron-carbon diagram and based on a high reeling temperature in the hot band. With rolling coefficients between 50 and 85%, this mixed grain leads to a lower tendency to form earing; however it also leads to the formation of course, band-shaped cementite, which causes undesirable structures on the steel sheet surface during the drawing of thin parts with high surface requirements, and, therefore, causes a high rate of defective products.
The task of this invention is to develop a general procedure leading to material properties, as for its anisotropy, very close to those of materials produced by normal annealing, while allowing relatively low operation costs with as few production steps as possible. The annealing process is supposed to produce a globular grain material; furthermore, the steel band produced by the invented procedure must show no disadvantages based on ageing or higher mechanical values due to high rolling coefficients.
According to this invention, the procedure of the aforementioned type suggests that the procedure steps performed after hot rolling include:
pickling
one- or multiple-stage cold rolling
annealing of the band in coiled state (coil annealing)
possibly also temper rolling of the band.
The warm band preferably contains boron in a portion between 0.0013 and 0.006 weight %, where the weight ratio of boron to carbon is from 0.5 to 2.5. The preferred goal should be to achieve a content of boron between 0.0013 and 0.003 weight %.
In order to achieve a uniform structure of the band material, hot rolling procedure is applied, preferably with the rolling temperature of over 870xc2x0 C. and a reeling temperature under 710xc2x0 C.
In order to achieve a very small earing formation during the deep drawing or drawing and ironing, and especially a relative earing of a maximum of 2.5%, the value of the vertical anisotropy xcex94r of the band after coil annealing should not amount to more than +/xe2x88x920.12.
Finally, this invention proposes a steel band capable to be processed by a deep drawing or drawing and ironing process, which is produced in a procedure according to at least one of the patent claims.